This invention relates to an improved type of stainless steel suitable for use in certain applications, such as in hydraulic machines which are required to have high cavitation erosion resistance and to the approaches for manufacturing or repairing such machinery.
Generator capacity in thermal and atomic power generators has recently become larger and larger, but it is difficult even for thermal and atomic power plants having these increased capacities to meet peak demands for electric power. One method for accommodating such a peak demand involves the construction of hydroelectric plants capable of rapidly changing their output and in particular the construction of pumped-storage power plants which use excess electric power at off peak periods to pump water to an elevated reservoir. The water remains in the reservoir until electric power is needed, at which times the water is passed through turbine generators.
The turbine usually employed in these pumped-storage power plants is the so-called "reversible pump turbine" type which generates electric power by day and pumps water by night. These power plants are being designed to operate with increasingly higher pressure heads and correspondingly higher outputs for the purpose of more efficiently using available building space and to reduce construction costs per unit output.
Cast steel (13-chromium cast steel, etc.) containing approximately 13 percent chromium by weight has conventionally been used as material for water turbine elements such as water turbine runners, guide vanes and stay vanes. Operating conditions, however, have become more and more severe. Consequently, the high velocity of water flow causes the formation of cavities on the surfaces of runner blades. When these cavities collapse on the runner blade surface they create repeated impulsive loads to be generated thereby damaging the surfaces of those runner blades. This is the so-called "cavitation erosion" that conventional materials cannot sufficiently resist.
Austenitic stainless steel, which has higher cavitation erosion resistance than the conventional 13-chromium cast steel, may be used as the manufacturing material for water turbine elements. The low proof stress of austenitic stainless steel permits it to be formed on cavitation erosion damaged surfaces by a manner of overlap welding.
Japanese Patent Disclosure No. 75984/80 discloses a stainless steel welding rod with cavitation erosion resistance comprised primarily of Cr and Ni with the Ni content being more than 7%, and the Cr content being more than 16%. Added to that Cr-Ni mixture is Co in an amount varying from 4 to 8%, and the final structure of the mixture exhibits the austenite and ferrite phases.
However, higher tread pressures and increased output capabilities have created a need for a material having superior cavitation erosion resistance from which turbine parts can be fabricated.